498 



NERVE. 



from the new poise caused by the current. The anodic rise and cathodic 

 fall both commence at the poles, on the moment of cessation with no 

 appreciable delay ; they probably extend from their polar seats of origin 

 into extrapolar regions, although this has not been satisfactorily proved. 

 The anodic polar rise is the most obvious ; this is probably due to the 

 fact that such opening effects are more demonstrable after long closures, 

 and with these the anodic condition is always accentuated. Hence, 

 during such a closure, the anodic fall is one which has to a great extent 

 swamped the cathodic rise in consequence of its greater development, so 

 that, on opening, the resulting anodic " rebound " is very appreciable, 

 and a rise of excitability ensues, which increases to a maximum and 

 then declines. 



On the other hand, the circumstances which favour the cathodic 

 rebound are, cccteris paribus, those which favour the production of 

 the preceding cathodic rise ; since, however, this rapidly attains its 

 maximum and then subsides even during closure, the physiological 

 condition which constitutes the rebound may be said to have begun 

 even before the cessation of the current flow ; hence, when such cessa- 

 tion occurs, the rebound constituting a cathodic fall of excitability is 

 not a prominent feature. It is, however, demonstrable with closures 

 of short duration, particularly by the use of mechanical excitation in 

 the neighbourhood of the poles ; this method has been employed by 

 Tigerstedt with great success in the determination of the four polar 

 excitability changes. 1 



The whole cycle of change due to the passage of a current may be 

 summed up as follows : — 



Closure. 

 Anodic fall of excitability (i.e. greater 

 stability of molecular equili- 

 brium), established instantane- 

 ously, increasing slowly to a 

 maximum, and slowly declining. 



Cathodic rise of excitability (i.e. less 

 stable molecular equilibrium), 

 established instantaneously, in- 

 creasing rapidly to a maximum, 

 and subsiding quickly. 



Cessation. 

 Anodic rise of excitability (rebound 

 to lessened stability), established 

 instantaneously, increasing, and 

 subsiding slowly. 2 



Cathodic fall of excitability (rebound 

 to greater stability), established 

 instantaneously and subsiding 

 rapidly. 



It has been suggested that the causation of the changes on 

 cessation is to be found in the reversed or polarisation current, which 

 is present in the nerve, and which is unmasked when the polarising 

 current ceases; this, if present, would turn the former anode into a 

 cathode, and vice versd. The changes on cessation would thus be, both 

 as to causation and character, simply those due to the closure of a 

 reversed polarisation current, which is unmasked by the cessation of 

 the polarising one. This simple explanation has been applied, as it 

 was logically compelled to be, to the explanation of the opening 

 excitation as well as the excitability changes. 3 There are, however, 



1 Tigerstedt, loc. cit. 



2 From experiments on the excitability produced on the nerves of man, Waller has con- 

 cluded that the anodic fall produced by a closure may last for "03 sec. after the current 

 has been opened, Arch, dephysiol. norm, etpath., Paris, 1882, p. 383. 



3 Tigerstedt, Mitth. v. physiol. Lab. d. Carolin. mcd.-chir. Inst, in Stockholm, 

 "Biological Memoirs," Oxford, 1887, vol. i. p. 49. 



